This invention relates to the construction of liquid crystal display devices. In particular, the invention is concerned with a reflection-type liquid crystal display device and a transflective liquid crystal display device of single polarizing film type, for effecting bright display in black and white or in color by means of a reflector and a sheet of polarizing film provided in a liquid crystal element of the device.
For a reflection-type liquid crystal display device, there has been mainly adopted a construction wherein a TN (twisted nematic) liquid crystal element or a STN (supertwisted nematic) liquid crystal element is disposed between a pair of polarizing films, and a reflector is installed on the outer side of one of the polarizing films.
With such a reflection-type liquid crystal display device, however, external light passes through each of two sheets of the polarizing films twice from the time when the external light enters from the visible side of the device until it goes out towards the visible side after reflected by a reflector, so that reduction in light quantity is increased, thereby lowering brightness of images in display. Moreover, since the reflector is installed on the outer side of a glass substrate of the liquid crystal element, there has arisen a problem that shadows appear on display.
To cope with the problem, a single polarizing film type liquid crystal display device, capable of effecting display with just one sheet of polarizing film, has since been proposed. With such a liquid crystal display device having only one sheet of polarizing film, reduction in light quantity can be decreased in comparison with the case of a conventional reflection-type liquid crystal display device employing two sheets of polarizing films, thereby improving brightness of images in display.
Further, with the single polarizing film type liquid crystal display device, it is possible to solve the problem of the shadows appearing on display by forming a reflector inside a liquid crystal display element.
Such a single polarizing film type liquid crystal display device is comprised of one sheet of polarizing film, one sheet of retardation film, and a liquid crystal element incorporating a reflector, as disclosed in, for example, Japanese Patent Laid-Open Publication No. 4-97121 (JP, 04-97121, A).
With such a conventional single polarizing film type liquid crystal display device as described above, however, a problem has been encountered that excellent black display can not be effected, and contrast becomes low.
In order to effect excellent black display, a low reflectance (a ratio of an outgoing light quantity to an incident light quantity as seen from the visible side) needs to be achieved in black display parts at all wavelengths in the visible light region. However, with the single polarizing film type liquid crystal display device, employing one sheet of retardation film described above, a low reflectance can be achieved for light rays at specific wavelengths, but it is impossible to achieve a low reflectance for light rays over all wavelengths.
Accordingly, there has since been developed a single polarizing film type liquid crystal display device employing two sheets of retardation films for effecting excellent black display, however, it is still a long way off from obtaining good contrast.
Further, there has since been developed a single polarizing film type liquid crystal display device, employing a compensating layer having an orientation rotated in the direction opposite to a twist direction of a liquid crystal layer, in place of the retardation film as disclosed in, for example, Japanese Patent Laid-Open Publication No. 10-123505 (JP, 10-123505, A). Even with such a construction, it has been difficult to achieve a low reflectance over all wavelengths, and consequently, good contrast has not been obtained.
Furthermore, with the conventional single polarizing film type liquid crystal display device of described in the foregoing, it is not possible to install a backlight because the reflector does not allow light rays to pass therethrough, so that display can not be seen at places where external light is weak or at night.
Accordingly, there has been developed a transflective liquid crystal display device, employing a half mirror made of a thin film of aluminum formed by the vapor deposition method or the sputtering method, or having a reflector provided with an opening for every pixel, so that display is effected by light rays emitted from a backlight at places where external light is weak or at night.
In the case of the single polarizing film type liquid crystal display device, at the time of reflective display using external light when incident light passes through a liquid crystal element back and forth, the liquid crystal element and optical elements such as a retardation film, and so forth, need to be designed such that display in excellent black and white can be obtained by controlling outgoing of reflected light with a sheet of the polarizing film.
On the other hand, at the time of transmissive display using a backlight, since light emitted from the backlight passes through the liquid crystal element only once, the liquid crystal element and the optical elements need to be designed such that display in excellent black and white can be obtained in such a condition as described above by controlling outgoing of reflected light with one sheet of the polarizing film. For these reasons, it has been difficult to obtain high contrast in both reflective display and transmissive display.
A liquid crystal display device having a reflector provided with an opening for every pixel has been disclosed in, for example, Japanese Patent Laid-open No. H 10-282488 (JP, 10-282488, A), however, no description on the conditions concerning a liquid crystal element and optical elements has been given therein at all, and no description on how to achieve good contrast at the time in both reflective display and transmissive display has been given therein at all.
The invention has been developed in view of the technical background described above, and an object of the invention is to provide a single polarizing film type liquid crystal display device to realize bright display in high contrast by obtaining excellent black display at low reflectance for light rays over all wavelengths.
Further, it is another object of the invention to provide a single polarizing film type liquid crystal display device, capable of effecting reflective display by use of external light and transmissive display by lighting up a backlight, and achieving high contrast at the time in both reflective display and transmissive display.
In order to achieve the objects described above, the liquid crystal display device according to the invention comprises a liquid crystal element comprised of twisted nematic liquid crystal sandwiched between a first substrate provided with a reflector and first electrodes and a second substrate provided with second electrodes, and a twisted retardation film, a first retardation film, a second retardation film, and a polarizing film, that are installed on the outer side (the side facing away from the twisted nematic liquid crystals) of the second substrate in sequence from the second substrate side towards the visible side, thus constituting a reflection-type liquid crystal display device of single polarizing film type.
With the liquid crystal display device, it is desirable that a twist direction of the twisted retardation film is opposite to a twist direction of the liquid crystal element, a twist angle of the twisted retardation film is substantially equal to a twist angle of the liquid crystal element, preferably the twist angle of the twisted retardation film is slightly greater than the twist angle of the liquid crystal element, and a xcex94nd value indicating birefringent tendency of the twisted retardation film is substantially equal to a xcex94nd value of the liquid crystal element.
Further, a phase delay axis of the first retardation film preferably crosses a phase delay axis of the second retardation film substantially at right angles, and wavelength-dependency of retardation value of the first retardation film preferably differs from wavelength dependency of retardation value of the second retardation film.
Or the phase delay axis of the first retardation film may cross the phase delay axis of the second retardation film so as to form substantially an angle of 60xc2x0 therebetween, and the retardation value of the first retardation film may be substantially equivalent to a quarter-wavelength while the retardation value of the second retardation film may be substantially equivalent to a half-wavelength.
Further, with any of these liquid crystal display devices, color filters may be installed either on the nematic liquid crystal side of the first substrate than the reflector, or on the nematic liquid crystal side of the second substrate, thereby constituting a color liquid crystal display device.
With any of these liquid crystal display devices, a diffusion film is preferably installed on the outer face of the second substrate.
The first electrodes may be formed of a reflective material so as to double as the reflector, rendering it unnecessary to install the reflector separately.
The liquid crystal display device according to the invention may comprise a liquid crystal element comprised of twisted nematic liquid crystal sandwiched between a first substrate provided with a transflective reflector and first electrodes and a second substrate provided with second electrodes, and a twisted retardation film, a first retardation film, a second retardation film, and a first polarizing film, that are installed on the outer side (the side facing away from the twisted nematic liquid crystals) of the second substrate in sequence from the second substrate side towards the visible side, together with a third retardation film, a second polarizing film, and a backlight that are disposed in sequence on the outer side of the first substrate, wherein retardation value of the third retardation film are substantially equivalent to a quarter-wavelength, thereby constituting a transflective liquid crystal display device of single polarizing film type.
Further, it is desirable that a fourth retardation film is installed between the third retardation film and the second polarizing film, a phase delay axis of the third retardation film crosses a phase delay axis of the fourth retardation film so as to form substantially an angle of 60xc2x0 therebetween, and the retardation value of the third retardation film are substantially equivalent to a quarter-wavelength while retardation value of the fourth retardation film are substantially equivalent to a half-wavelength.
Otherwise, the phase delay axis of the third retardation film may cross the phase delay axis of the fourth retardation film substantially at right angles, wavelength-dependency of the retardation value of the third retardation film may differ from wavelength-dependency of the retardation value of the fourth retardation film, and the difference between the retardation value of the third retardation film and the retardation value of the fourth retardation film may be substantially equivalent to a quarter-wavelength.
Also, it is desirable that the twist direction of the twisted retardation film is opposite to the twist direction of the liquid crystal element, the twist angle of the twisted retardation film is substantially equal to the twist angle of the liquid crystal element, preferably the twist angle of the twisted retardation film is slightly greater than the twist angle of the liquid crystal element, the xcex94nd value indicating birefringent tendency of the twisted retardation film is substantially equal to the xcex94nd value of the liquid crystal element, the phase delay axis of the first retardation film crosses the phase delay axis of the second retardation film so as to form substantially an angle of 60xc2x0 therebetween, and the retardation value of the first retardation film are substantially equivalent to a quarter-wavelength while the retardation value of the second retardation film are substantially equivalent to a half-wavelength.
Or the phase delay axis of the first retardation film may cross the phase delay axis of the second retardation film substantially at right angles, wavelength-dependency of the retardation value of the first retardation film may differ from wavelength-dependency of the retardation value of the second retardation film, and the difference between the retardation value of the first retardation film and the retardation value of the second retardation film may be substantially equivalent to a quarter-wavelength.
Further, with any of these liquid crystal display devices, color filters may be installed either on the nematic liquid crystal side of the first substrate than the reflector, or on the nematic liquid crystal side of the second substrate, thereby constituting a color liquid crystal display device.
Further, with any of these liquid crystal display devices, a diffusion film is preferably installed on the outer face of the second substrate.
Furthermore, the transflective reflector may be composed of a thin metal film having a thickness in a range of 0.01 to 0.03 xcexcm. Otherwise, the transflective reflector may be composed of a thin metal film provided with an opening at every spot corresponding to respective pixels.
As described in the foregoing, for the liquid crystal display device according to the invention, one sheet of the twisted retardation film and two sheets of the retardation films are used as optical elements of a single polarizing film type liquid crystal display device of. By causing the twist direction of the twisted retardation film to be opposite to the twist direction of the liquid crystal element, and by causing the twist angle and the xcex94nd value indicating birefringent tendency of the twisted retardation film to be substantially equal to the twist angle and the xcex94nd value of the liquid crystal element, it is possible to fully compensate birefringency of the liquid crystal element.
Further, with the use of two sheets of the retardation films, it becomes possible to change wavelength-dependency of retardation values, thereby enabling a so-called wide-band quarter-wavelength film to be formed. With the wide-band quarter-wavelength film, retardation values in a short-wavelength region become smaller, and retardation values in a long-wavelength region become larger. As a result, the quotient F/xcex, a retardation value F divided by a wavelength xcex can be rendered approximately one quarter over all wavelengths.
It follows therefore that, in effect, the wide-band quarter-wavelength film is disposed on top of the reflector, and the polarizing film is disposed on top of the wide-band quarter-wavelength film. If the polarizing film is disposed such that a transmission axis thereof is at 45xc2x0 relative to the phase delay axis of the wide-band quarter-wavelength film, incident linearly polarized light is turned into circularly polarized light at all wavelengths, and upon passing through the wide-band quarter-wavelength film again after reflected by the reflector, the circularly polarized light is turned into linearly polarized light with its direction of polarization rotated through 90xc2x0 at all wavelengths, which is then absorbed by the polarizing film, thereby indicating perfect black display.
That is, at the time of reflective display, by use of one sheet of the twisted retardation film and two sheets of the retardation films, the birefringent tendency of the liquid crystal element can be fully compensated by the twisted retardation film, and reflectance at all wavelengths can be lowered by forming the wide-band quarter-wavelength film with the two sheets of the retardation films. As a result, excellent black display can be obtained, thus enabling display in high contrast to be effected.
Meanwhile, in the case of the transflective liquid crystal display device, operation at the time of reflective display is the same as that of the reflection-type liquid crystal display device described in the foregoing. However, at the time of transmissive display, light emitted from the backlight is transmitted through the polarizing film and the retardation film having retardation value equivalent to a quarter-wavelength which are installed on the backside of the liquid crystal element, and is further transmitted through the transflective reflector before falling on the liquid crystal element. Since birefringent tendency of the liquid crystal element is fully compensated by the twisted retardation film, light undergoes no change in polarization state even when passing through the liquid crystal element and the twisted retardation film, and falls on the wide-band quarter-wavelength film provided on the visible side of the liquid crystal element.
By disposing the wide-band quarter-wavelength film provided on the visible side of the liquid crystal element such that retardation value thereof is subtracted from retardation value of the retardation film installed on the backside of the liquid crystal element, the light emitted from the backlight arrives at the polarizing film on the visible side as it is. Accordingly, if the polarizing film on the backlight side and the polarizing film on the visible side are disposed such that respective transmission axes cross each other at right angles, excellent black display can be effected.
In a state where a voltage is applied to the liquid crystal element, the birefringent tendency of the liquid crystal element undergo change, so that excellent white display can be effected at the time of reflective display as well as the time of transmissive display, thereby enabling display in high contrast to be obtained at both the time of reflective display and the time of transmissive display.